PropertyLink

In addition to the scalar Properties of an feature, the features themselves contain pointers to one another. These pointers define a directed acyclic graph that determines the set and ordering of objects that are recomputed in response to a change in one object. Only those features that depend on a changed feature are recomputed.

The dependencies are expressed via a special class of Property types, namely the PropertyLink:

PropertyLink: this allows a feature to link to another single feature within the same document.
PropertyLinkList: this allows a feature to link several features
PropertyLinkSub: this allows a feature to link a single feature and additionally reference sub-elements. Example: If you want to model a pocket for the needed sketch, then it's important to know on which sub-element (e.g. Face6) of the linked feature it must be mapped to.
PropertyLinkSubList: this allows a feature to link to several sub-elements of several features.

The following are similar properties for linking features of different documents. This is the core part for assemblies.

PropertyXLink
PropertyXLinkSub
PropertyXLinkSubList
PropertyXLinkList
PropertyXLinkContainer

Example

Consider a class BoxDimension that provides basic dimensions for another class Box. We would like a Box object to be recomputed whenever its associated BoxDimension is changed:

import FreeCAD
import Part

class BoxDimension:
  def __init__(self, obj):
    obj.addProperty("App::PropertyLength", "Length").Length = 1
    obj.addProperty("App::PropertyLength", "Width").Width = 1
    obj.addProperty("App::PropertyLength", "Height").Height = 1
    obj.Proxy = self

class Box:
  def __init__(self, obj):
    obj.addProperty("App::PropertyLink", "Dimensions").Dimensions = None
    obj.Proxy = self
  def execute(self, obj):
    if obj.Dimensions is None:
      return
    l = obj.Dimensions.Length
    w = obj.Dimensions.Width
    h = obj.Dimensions.Height
    obj.Shape = Part.makeBox(l, w, h)

Note that it is a Box object that contains the PropertyLink to the BoxDimension object. Usage is as follows:

doc = App.newDocument()
dim = doc.addObject("App::FeaturePython", "BoxDimension")
box = doc.addObject("Part::FeaturePython", "Box")
dim_proxy = BoxDimension(dim)
box_proxy = Box(box)
box.ViewObject.Proxy = None
box.Dimensions = dim

dim.Length = 5
dim.Width = 3
dim.Height = 7
doc.recompute()

Because our box depends on the dim object, it will be recomputed.

InList and OutList

PropertyLink objects can be accessed using a Python property using the name that they are registered with using .addObject(). However there is another way. Every feature has a pair of lazily-generated lists called InList and OutList that describe the outgoing and incoming edges of the DAG, respectively:

An InList is a list of all features that depend upon the current object. So, dim.InList will be a list containing our box object.
Similarly, an OutList is a list of all features that are depended upon the current object. That is, box.OutList will be a list containing our dim object.

Note that InList and OutList have nothing to do with the tree view of the document model that is presented in the GUI. At any time, a parent in that tree view may contain children that are part of the InList, the OutList, or neither.